William was the eldest son of Henry,
Baron Norreys of Rycote, and his wife, Margery the daughter of John,
Baron Williams of Thame. He would have mostly grown up at Wytham
Abbey. Later, as a young man, he spent some time in France,
where his father was ambassador, and some in the household of Sir William
Cecil in London.

William and his brother, John,
were eventually recommended to Walter Devereux, Earl of Essex, by their
father and they travelled with him on his expedition to conquer Ulster and
colonize Ireland in 1573. William became the Earl's chief lieutenant and
commanded a troop of 100 mounted men. The Irish used guerilla tactics
against the English invaders and William was the first of Essex's commanders
to be ambushed by them, near Massareene. His horse was killed beneath him
and he would have been killed, had his brother not rescued him. The Earl
praised their actions in a letter to the queen.

Back in England, in 1576,
William married Elizabeth, daughter of Sir Richard Morrison of Cassiobury in Hertfordshire,
and was temporarily appointed Marshal of Berwick in succession to Sir
William Drury, but he soon returned to Ireland, hoping to build a Norreys
'empire' in the Emerald Isle. However, it was not to be. William did obtain
a knighthood and became Receiver of Ireland in the Irish Government after
the Earl of Essex's death at which he was present.; but he caught a violent
fever and died at Newry on 25th December 1579. It is said that he accurately
foretold his own death. Queen Elizabeth sent his mother a letter of
condolence, but this can have been little comfort to his widow who was now
the mother of a four month old son, Francis, later to become Earl of
Berkshire.